I'll add to the above with a few pointers.
Are the brand of tyres the same? A more sticky tyre should in theory reduce MPG.
Also, are the new tyres wider than the old 18s? Again, with more rubber in contact with the road, you've got more drag, therefore poorer MPG.
In addition, is the rolling radius actually the same (ie. are you sure you've got the correct profile ratio on the new tyre)? As above, a bigger diameter will not only give a bigger rotational mass, but also a longer contact patch on the road, which will as before reduce MPG.
On top of that, the newer wheels themselves may be just a bit heavier - that extra inch diameter of metal could well weigh a bit more. Again you're into increased rotational mass there.
Finally (phew!), check your tyre pressures - low pressures can reduce MPG.